1. Field of the Invention
This invention relates to a magnetic rotary encoder, and more particularly to an apparatus of the kind described above which uses a magnetoresistive element.
2. Prior Art of the Invention
It is commonly known that, when a magnetoresistive element made of a magnetoresistive material such as permalloy is placed in a magnetic field and current is supplied thereto, the resistance value of the magnetoresistive element varies depending on the intensity of the magnetic field. Thus, when such a magnetoresistive element is used in lieu of a magnetic head to sense magnetic information recorded on a magnetic recording medium, the recorded magnetic information can be reliably sensed even when the relative velocity of the magnetic recording medium and the magnetoresistive element is zero, that is, even in a stopped condition of the magnetic recording medium. It is also commonly known that such a principle can be utilized to sense the velocity of rotation or angular position of rotation of a rotating body by the magnetoresistive element. Suppose, for example, that serial pieces of magnetic information are recorded on a circumferential track provided on the surface of a rotary magnetic recording medium, and the magnetoresistive element is disposed to extend in the radial direction of the magnetic recording medium with its sensing portion brought close and opposite to the magnetic information recorded on the magnetic recording medium. In such a case, one of the lead connection terminal portions connected to the respective ends of the sensing portion of the magnetoresistive element can be disposed outside of the outer periphery of the rotary magnetic recording medium without any difficulty. However, in this arrangement, the other lead connection terminal portion of the magnetoresistive element is necessarily situated at a position opposite to the associated surface portion of the magnetic recording medium, and a lead is fixed thereto by means such as soldering. The arrangement above described in therefore defective in that the desired close approach of the magnetoresistive element toward the magnetic recording medium is obstructed by the amount corresponding to the protuberant head of solder. It is apparent that the sensor sensitivity increases as the sensing portion of the magnetoresistive element is situated closest possible to the magnetic information recorded on the magnetic recording medium.
When serial pieces of magnetic information are to be recorded on a recording medium such as a plastic magnet, it is difficult to record the information at a high recording density by being limited by the recording capability of a conventional recording unit, and a length of about 1 mm is generally required for recording one piece or unit of information according to the present technique of information writing. Thus, for example, information as many as about 100 pulses can only be recorded on a circumferential track when the plastic magnet is cylindrical in shape and has a diameter of 40 mm. Therefore, in order to read the recorded information with a higher resolution during rotation of the plastic magnet, that is, in order to improve the resolution, it is known to radiately dispose a plurality of magnetoresistive elements in a relatively phase-shifted relation and opposite to magnetic information recorded on a recording medium. Such an arrangement is disclosed, for example, in Japanese patent application No. 51-145148 (filed on Dec. 2, 1976 and laid-open for inspection on June 21, 1978 as patent application Laid-Open No. 53-69684). The cited Japanese laid-open application discloses a rotation velocity sensing apparatus in which a serial pieces of magnetic information are recorded on the surface of a rotating disc, and a plurality of magnetoresistive elements spaced apart by a predetermined distance from each other are disposed close and opposite to the magnetic information recorded on the disc so that plural velocity information signals having a phase difference therebetween can be derived from the individual elements. In this case too, the protuberant head of solder soldering a lead to the radially inner lead connection terminal portion of each magnetoresistive element provides also a hindrance to the desired improvement in the sensor sensitivity, as pointed out hereinbefore. In addition, the necessity for the positioning of the plural magnetoresistive elements at the positions spaced apart by the predetermined distance poses a problem from the viewpoint of accuracy of their positions. When the mounted positions of the magnetoresistive elements deviate from the predetermined positions, the output pulses will not have the same pulse interval or the output sensitivity will be degraded. Especially, when the width of each piece of recorded information on the magnetic recording medium is narrowed to increase the number of output pulses thereby improving the resolution or sensor sensitivity, the space between the magnetoresistive elements must also be narrowed to meet the narrowed width of each piece of the recorded information. However, the disclosed apparatus is still defective in that there is a limitation in the accuracy of positioning the magnetoresistive elements, and the number of output pulses cannot be increased over some limit.